1. Field of the Invention
This invention relates to a mimeographic printing machine in which a rotary plate cylinder supports on its outer circumferential surface a stencil and transfers an original image to a paper sheet with ink supplied from inside the plate cylinder through pores of the stencil.
2. Description of the Related Art
A mimeographic printing machine is known in which a rotary plate cylinder supports on its outer circumferential surface a stencil and transfers an original image to a paper sheet with ink supplied from inside the plate cylinder through pores of the stencil.
In such a printing machine, as shown in FIG. 14 of the accompanying drawings, an ink supply mechanism located in the plate cylinder includes a squeegee roller 101 and a doctor roller 102 in contact with the squeegee roller 101.
With this ink supply mechanism, it is very difficult to adjust a clearance between the squeegee roller 101 and the inner circumferential surface of the plate cylinder. Therefore it would be further difficult to regulate the amount of ink to be supplied. The inner circumferential surface of the plate cylinder 100 and the outer circumferential surface of the squeegee roller 101 are in contact with each other tangentially to squeeze the ink more than necessary. The excessive ink gathers at a perforation-free portion, i.e. a trailing end, of the stencil. Then the gathered ink overflows from the trailing end of the stencil to smear the paper sheet.
It is conceivable to increase the viscosity of the ink so as to overcome the inconvenience encountered with the prior art. Having an increased viscosity, however, the ink hardly permeates into the tissues of the paper sheet, which would result in an insufficient printing density.
In an attempt to overcome these prior problems, a mimeographic printing machine has been developed, in which the squeegee is located inside the plate cylinder. In use, the squeegee is brought into contact with the plate cylinder and is rotated with the plate cylinder. Ink is supplied to the inner circumferential surface of the plate cylinder and is then squeezed toward the stencil wound on the outer circumferential surface of the plate cylinder.
In the second-named printing machine, the squeegee is in direct contact with the inner circumferential surface of the plate cylinder. Therefore it is unnecessary to adjust a small clearance between the squeegee and the inner circumferential surface of the plate cylinder. Even if low-viscosity ink is used, it is possible to control the amount of ink to be supplied to the outer circumferential surface of the plate cylinder, because the squeegee is substantially in linear contact with the inner circumferential surface of the plate cylinder. Also the overflow of ink can be minimized effectively.
It is however still impossible to perfectly prevent the overflow of ink. The overflow of ink may be further minimized in the manner shown in FIG. 15. As shown in FIG. 15, a squeegee 103 does not serve to squeeze the ink to a perforation-free portion 104 of the stencil beyond the perforation of the stencil. Specifically, the squeegee 103 is moved upwardly once in front of the perforation-free portion 104, as indicated by an arrow in FIG. 15. For the next printing, the squeegee 103 is brought into contact with the inner circumferential surface of the plate cylinder at the perforation-free portion 104 in front of the perforation.
The overflow of ink can be prevented by the arrangement of FIG. 15, leaving the following difficulties unsettled.
Firstly, ink is supplied to an ink reservoir 105 defined between the inner circumferential surface of the plate cylinder 100 and the squeegee 103. As the plate cylinder 100 rotates, the squeegee 103 forces the ink against the inner circumferential surface of the plate cylinder 100 to push the ink toward the outer circumferential surface of the plate cylinder 100.
On the contrary, as shown in FIG. 16(a), when the squeegee 103 descends from the lifted position and starts contacting with the plate cylinder 100, there is no ink reservoir to be formed between the squeegee 103 and the inner circumferential surface of the cylinder 100. Specifically, the squeegee 103 starts contacting the inner circumferential surface of the plate cylinder at the trailing or upstream end of the perforation-free portion 104 of the plate cylinder 100. A small ink reservoir 105 may be formed while the squeegee 104 squeezes the ink toward the perforation 106 of the plate cylinder 100 as shown in FIG. 16(b).
However, since the squeegee 103 squeezes all of the ink from the ink reservoir 105 at the leading edge of the perforation 106 as shown in FIG. 16(c), it takes a certain period of time to form the ink reservoir 105 again for the next squeezing operation as shown in FIG. 16(d). Therefore the most freshly printed image has a normal density at its leading edge, a lower density at the central portion, and a normal density at the trailing end. In other words, the printed image would suffer non-uniform density.